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Full text of "Flora Indica Vol-I"

INTRODUCTORY   ESSAY.                                217
and commence again in the middle of September. A. rather
sudden rise of temperature attends the vernal equinox, and
the summer is comparatively warm, the maximum sometimes,
but rarely,, reaching 70.
At 13,000 feet the mean temperature probably coincides
with that of the freezing-point. At 14-15,000 feet the sum-
mer months alone are free from night-frosts, the maximum
temperature is only 60 in good shade, and the winter is pro-
portionately colder than at 12,000 feet; thaw commences at
the end of April, the night-frosts are slight by the end of that
month, and the mean of the day rises to 50. At 15,500 feet
it probably freezes during every night of the year. At 20,000
to 21,000 feet there is probably perpetual frost in. the shade.
These numbers however give no indication of the heat to
which vegetation is exposed, for, owing to the rarity of the
atmospherg and cloudless skies, the sun's rays have intense
power, increasing with the elevation, raising the (white glass)
thermometer exposed to them sometimes upwards of 100
above the mean temperature of the air. This, combined with
the fact of the temperature of the soil being always above that
of the aur, fully accounts for the sudden impulse given in
spring to the vegetation even in the loftiest and coldest re-
gions. The heat .radiated from the naked rocks has also a very
powerful effect, especially on the summer crops.
Extreme aridity is the characteristic of all Western Tibet,
Bain and snow at moderate elevations are scarcely known,
and have no further direct effect on vegetation than is due to
the moisture of the soil produced by the melting of glaciers
and snow-beds. Dew and hoar-frosts are very rare pheno-
mena. The snow-level is nowhere below 18,000 feet; in the
mountains north of the upper Indus valley it rises to 20,000.
Owing mainly to the great drought, the soil is in many
places covered with an efflorescence of carbonate and other
salts of soda, and salt-lakes are of frequent occurrence. Almost
all the large bodies of water indeed are more or less saline,
some of them intensely so, especially such a& liave no outlet,
a/